Conventionally, it has been very difficult to correct the gradation of an image having extremely bright and dark portions. In other words, gradation-correction based on the bright portion causes the dark portion to blacken, making only the dark portions conspicuous and placing the entire image in poor gradation. On the contrary, gradation-correction based on the dark portion causes the color of the bright portion to be too white and the gradation to disappear. As a result, the bright portion will lack gradation and the entire image will be in poor gradation.
To reduce such a difference in brightness, a conventional gradation-conversion-processing method uses a low-contrast curve. It is difficult, however, to drastically reduce an extremely large difference in brightness in an image. Moreover, attempts to perform such a reduction tend to decrease differences in brightness in the middle gradation area of the image.
Another conventional processing method makes uniform a distribution-of-graduation histogram of an image in order to correct the image with a biased gradation. In this processing method, it is possible to increase the difference in brightness in the dense area of the gradation histogram. However, in the sparse area of the gradation histogram the difference in brightness is reduced, which may result in the gradation collapse becoming conspicuous at some portions of the image.
In the technique disclosed in U.S. Pat. No. 5,991,456, it is possible to increase the brightness of a shadow area of an image and to enhance a small gradational change in the shadow area. As a result, it is possible to brighten a gradational change, which would otherwise be hidden in the shadow area of the image. But, due to the brightened shadow area, color balance of the shadow area may appear unnatural, and blue components of the shaded portion may be enhanced, for example. Further, a too-brightened shadow area can impair the shades of the entire image, giving the image a somber impression.